How To Choose Brake Fluid And Bleed Brakes

Keeping Your Mustang's Braking System In Proper Working Order Calls For Regular Preventative Maintenance

Jim Smart

August 1, 2008

All the horsepower and good looks in the world are meaningless without safe brakes. To have good brakes, you must stay on top of regular preventative maintenance in every respect because they need tuning up periodically just like engines. While engines need spark plugs, fuel filters, and oil changes, brake systems require fresh pads, turned rotors and drums, replaced seals, and the periodic change of fluid. Every so often, we need to reline brakes and machine rotors and drums, which gets brakes back on top of their game. But hydraulic brakes should have more than just good pads and machined surfaces. They need clean, uncontaminated fluid to do the best job.

Brake fluid contaminants include air, moisture, solvents, dirt, rust, rubber, and a host of other elements. Hydraulic brake fluid must be able to withstand high temperatures during hard braking and still be able to stop the car-and it can't freeze when the mercury plummets. It's also a lubricant, which means it has to protect moving parts. Clean fluid is necessary for safe operation. Flush and bleed brake fluid every two years regardless of mileage.

Where brake fluid differs from engine oil, transmission fluid, gear lube, or antifreeze is its status as a hygroscopic fluid. It absorbs moisture, even through steel lines, reinforced hoses, and capped master cylinders. Take the cap off your Mustang's master cylinder, and brake fluid takes on air and moisture, which can make unpleasant things happen under pressure.

Slamming hard on the brakes results in a lot of pressure and heat, which can boil contaminants in the fluid. Those moisture pockets become steam pockets, which generate air pockets, making your brake pedal pulsate during hard braking. Don't get this confused with a warped rotor or irregular drum, which also causes pedal chatter in rhythm with wheel rotation. With excess moisture in the brake fluid, hard braking will generate little pulses, or hammering, in the pedal, resulting in poor braking performance. The more moisture there is in the brake fluid, the lower the fluid's boiling point.

As little as 3 percent moisture in your Mustang's braking system can reduce its effectiveness by 30 percent. This is when your Mustang's braking system becomes dangerous because it won't stop you in a panic situation. In fact, it will struggle to stop under normal conditions.

Moisture in brake fluid also aggravates corrosion inside brake lines no matter how protected they are with galvanizing or protective coating. This makes high-pressure lines rust through, causing brake failure.

Brake failure typically happens without warning when a seal fails or a line bursts from fluid contamination, rendering your Mustang's braking system inoperative, especially if it's a '65-'66 with a single master cylinder. Sometimes brakes fail gradually as pads wear and fluid becomes more contaminated.

Also remember that brake fluid absorbs moisture in the bottle. Keep some on hand for brake servicing, but properly dispose of it every two years to keep your inventory fresh. It's a good idea to write the purchase date on the bottle to keep track.

The different types of brake fluid offer their own advantages and disadvantages. If you still have DOT 2 on the shelf, dispose of it because it was designed for older drum brake systems and has a much lower boiling point than DOT 3 or DOT 4. Any DOT 2 fluid still on the shelf has absorbed more moisture than you'd want in your braking system anyway.

Although DOT 3 and 4 are similar, we discourage mixing the two. If you have DOT 3, stick with it unless you're putting in all-new brake fluid. In that case, use DOT 4. The advantage of DOT 4 is its boiling point and synthetic makeup, making it less prone to moisture absorption. It must still be flushed and changed every two years.